The Colosseum is the most  recognizable structure in Rome. Once an ampitheatre for gladiator contests and spectacles, it could hold up to 50,000 people; truly an accomplishment for the time. Steps away is the Roman Forum, which served as the center of political and social activity in ancient Rome.
"[My favorite landmark was] the Colosseum. I believe I'm biased in my answer because I lived 15 minutes walking distance from the Colosseo Metro stop." -Dorian C., Wake Forest University